* Change compiler flags for Visual Studio v16.4.0 & v16.5.0

 - core/base: Since Visual Studio v16.5.0 the /O2 optimization flag makes most of the roofit/roostats tests failing
 - hist/histv7/test: Using /O2 compiler flag prevent the followin error when building in debug mode:
   axis.obj : fatal error LNK1179: invalid or corrupt file: duplicate COMDAT '??$?8DU?$char_traits@D@std@@@__ROOT@experimental@std@@YA_NV?$basic_string_view@DU?$char_traits@D@std@@@012@0@Z'